1. Field of the Invention
The present invention generally relates to charged particle beam tools and, more particularly, to real-time measurements for adjustments of charged particle beam tools, especially electron beam lithography systems.
2. Description of the Prior Art
Charged particle beam tools are known and used for numerous purposes including electron microscopy and lithographic exposures using charged particles such as for the manufacture of semiconductor integrated circuits. In the latter case, in particular, demands for increased integration density and reduction of size of elements of integrated circuits have required electron beam exposures of resists when deep ultraviolet light exposures have become inadequate to support minimum feature sizes required by current and foreseeable integrated circuit designs. Given the requirement for such high resolution in order to obtain the benefits of improved performance and functionality as well as increased manufacturing economy, high performance is required from charged particle (e.g. electron) beam exposure systems.
Electron beam exposure systems generally fall into two general types: probe-forming systems in which a single spot, generally of a selected shape, is exposed at a time at a high repetition rate and electron beam projection systems in which a sub-field pattern defined in a reticle sub-field is projected onto a target and sequential exposures of different sub-fields are made across the chip area. Electron beam projection lithography systems have much increased throughput relative to probe-forming systems by greatly increasing the number of pixels which can be simultaneously exposed. In both of these types of systems, high positional accuracy and shape fidelity are required to obtain the desired pattern of exposure, particularly to achieve proper stitching together of individual exposures.
While electron beam lithography systems have become highly sophisticated and are capable of high accuracy and fidelity through utilization of a number of electron-optical elements such as lenses, deflectors, dynamic correction elements and the like, none of these elements can be perfectly fabricated or fully modeled theoretically. Therefore, it is necessary to adjust and/or align the elements and their driving circuits or the beam relative to the elements to optimize performance of the tool. Some adjustments may be made in real-time and more-or-less elaborate protocols have been developed to achieve optimum alignment in regard to some operating parameters of the tool. Other parameters have presented persistent practical problems in making real-time adjustments based on real-time performance measurements.
When real-time measurements cannot be made, operation of the tool must be interrupted while a measurement is made through some other facility, such as withdrawing an exposed, resist-coated wafer from the tool, developing the resist and evaluating the patterned features formed in the resist. Of course, such interruption of operation is not only time-consuming but may introduce additional errors or sources of error into the adjustment of the tool.
Two key parameters of charged particle beam tool performance are telecentricity and landing angle. Landing angle should ideally be perpendicular to the target plane and deviations therefrom introduce field translation errors as a function of height of an image location from the plane of best focus. Deviation from telecentricity is manifested as variation of landing angle with deflection and can cause magnification and/or rotation of the deflected field as a function of height. These image aberrations can generally be quantitatively evaluated only by inspection of images in resist.
However, one attempt at a real-time measurement of deviations from telecentricity and perpendicular landing angle which has been attempted involved a grid mounted on a piezoelectric transducer which was intended to provide translation of the grid in the axial direction of the tool. The beam was scanned in an xe2x80x9cLxe2x80x9d shaped pattern in the x and y directions and the resulting differentiated backscattered beam was displayed on a spot monitor while the piezoelectric transducer was driven with a square wave pulse. The spot monitor could thus display the backscattered beam from the grid at the two axial locations of the grid and alignment of the electron beam could be adjusted with a goal of minimizing or eliminating the positional shift of the backscattered beam. This, in theory, could be used to confirm the landing angle of the beam. If the beam was further deflected, adjustments in deflection could, in theory, be made such that the landing angle was independent of deflection.
As a practical matter, this technique was unsuccessful due to an unavoidable component of lateral motion of the grid; from which deviations from telecentricity and/or perpendicular landing angle were indistinguishable from the grid translation. In other words, any lateral component of motion of the grid as the grid was driven in the z-direction would result in total grid displacement other than perpendicular to the target plane. Thus, adjustment to minimize positional changes in the image of the backscattered beam as the grid was moved would assure a corresponding deviation from telecentricity and landing angle to conform the beam trajectory to the grid motion.
The only solution proposed for this inherent problem was to apply mechanical stops to limit lateral grid motion. However, it proved impossible to constrain lateral grid motion to a level that would assure negligible influence on the result. The more recent increased accuracy requirements imposed by incresed integration density and reduced device size render this technique even more unacceptable for a real-time measurement of telecentricity and/or landing angle.
It is therefore an object of the present invention to provide a technique for measurement of telecentricity and landing angle of a charged particle beam in substantially real time to facilitate adjustment of a charged particle beam tool.
It is another object of the invention to provide for increased exposure accuracy and fidelity in electron beam lithography tools
In order to accomplish these and other objects of the invention, a charged particle beam system and method are provided including an arrangement or step for moving a block in a direction generally perpendicular to a target plane of the charged particle beam system, a grid carried by the block and capable of backscattering a portion of a pattern of charged particles incident thereon, optical means for measuring position on the block, a sensor arrangement for measuring backscatter of charged particles from the grid, and an arrangement for correlating charged particle backscatter with positions of the block and computing position of a charged particle beam at different positions of the block along an axis of said charged particle beam system.